Heat exchanger

ABSTRACT

A heat exchanger for the cooling of water by an airstream comprises a plurality of generally parallel hollow wall members forming vertical passages to be traversed by the cooling air, each wall member consisting of an envelope of thin plastic sheet material provided at the top with an inlet for hot water which is allowed to flow down on the inner shell surfaces in a continuous film of liquid. The envelope opens at the bottom into a pool of water and is sealed against the atmosphere to enable the maintenance therein of air under pressure inflating it against a reinforcing external grid of tensioned cables or the like.

United States Patent Meylan et al.

[4 1 Oct. 21, 1975 HEAT EXCHANGER [75] Inventors: Jean-Luc Ch. Meylan,Geneva;

William H. Frost, Grand-Laney, Geneva; John G. Meier, Geneva, all ofSwitzerland [73] Assignee: Battelle Memorial Institute,

Carouge, Geneva, Switzerland [22] Filed: Sept. 21, 1973 [21] Appl. No.:399,513

[30] Foreign Application Priority Data Sept. 22, 1972 Switzerland13899/72 [52] US. Cl 165/115; 261/.Ol1 [51] Int. Cl. F28d 9/00 [58]Field of Search ..261/.011, 104, 107, 112; 165/1l1,115,122,125, 129

[56] References Cited UNITED STATES PATENTS 3,371,709 3/1968 Rosenblad165/115 3,374,994 3/1968 Greor 3,412,778 11/1968 Witt et al 165/115 X3,811,661 5/1972 Procter 261/107 FOREIGN PATENTS OR APPLICATIONS 571,5108/1945 United Kingdom 261/112 Primary ExaminerCharles J. Myhre AssistantExaminerTheophil W. Streule, Jr. Attorney, Agent, or Firml(arl F. Ross;Herbert Dubno [57] ABSTRACT A heat exchanger for the cooling of water byan airstream comprises a plurality of generally parallel hollow wallmembers forming vertical passages to be traversed by the cooling air,each wall member consisting of an envelope of thin plastic sheetmaterial provided at the top with an inlet for hot water which isallowed to flow down on the inner shell surfaces in a continuous film ofliquid. The envelope opens at the bottom into a pool of water and issealed against the atmosphere to enable the maintenance therein of airunder pressure inflating it against a reinforcing external grid oftensioned cables or the like.

l0 Claims, 6 Drawing Figures US. Patent Oct. 21, 1975 Sheet 1 on3,913,667

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FIG.

U.S. Patent Oct.21, 1975 Sheet20f3 3,913,667

FIG. 2

HEAT EXCHANGER FIELD OF THE INVENTION Our present invention relates to aheat exchanger designed to establish thermal contact between two fluidsof different temperatures, at least one of these fluids being a liquid.In its more specific aspects it concerns a heat exchanger designed tocool large quantities of hot water, such as the effluents of electricpower plants or nuclear reactors, by thermal contact with theatmospheric air.

BACKGROUND OF THE INVENTION In modern industry, and especially in thefield of generating electric power, large quantities of heat areproduced which must be dissipated at low cost and without detrimentallyaffecting the environment, i.e., with a minimum rise in temperature ofan ambient medium into which this waste heat is discharged. In the past,rivers and lakes were rather extensively used for getting rid of hoteffluents; recent ecological studies, however, have shown the necessityfor imposing strict limitations upon this method of disposal.

An alternative way of dissipating waste heat is the utilization ofatmospheric air as a coolant. This may be conventionally accomplishedwith the aid of cooling towers of either wet or dry" type through whichthe air is forced to circulate by natural draft and/or with the aid ofblowers: In a wet tower the hot waste water is dispersed into the risingair stream for vaporization thereby; this is a relatively efficient wayof cooling, resulting in only a minor rise in air temperature, but hasthe drawback of introducing additional moisture into the atmospherewhich may lead to the formation of rather dense localized fog that caninterfere with nearby road or air traffic. In a dry tower the watercirculates through finned tubes which must have a large effectivesurface area and which entail considerably higher costs than wet towersof equivalent capacity; also, the rise in air temperature issubstantially higher in that instance.

OBJECTS OF THE INVENTION The general object of our present invention,therefore, is to provide an improved heat exchanger for the purpose setforth which avoids the disadvantage of earlier systems of this type.

A more particular object is to provide means in such a heat exchangerfor intensifying the thermal contact between two fluids, such as waterand air, while keeping them physically separated from each other.

SUMMARY OF THE INVENTION These objects are realized, in accordance withour present invention, by the provision of a plurality of spacedlyjuxtaposed hollow wall members defining passages between them for thecirculation of one of the thermally interacting fluids, such as anairstream. Each wall member includes a generally horizontal elevatedsupport and an envelope of plastic sheet material suspended therefrom,this envelope comprising a pair of confronting sheets separated by anairspace. A channel on the elevated support receives the other (liquid)fluid, such as hot water, via a supply conduit and discharges it throughgaps, leading to the aforementioned airspace, onto the inner surfaces ofthe sheets for distribution along upper edge portions thereof from whichthis fluid descends in a continuous film along the sheet surfaces into acollector at the bottom of the envelope. Thus, the liquid of the filminteracts thermally over virtually the entire envelope area with theexternally circulating air or other fluid.

Advantageously, each envelope is closed against the atmosphere at itstop and sides while dipping at its bottom into a pool of liquid held ina basin which constitutes the aforementioned collector, the liquid levelin that basin reaching above the envelope bottom so as to merge with theinternally descending liquid films and to complete the seal. Theinterior of the envelope can then be inflated by air or some inert gasunder pressure, preferably against an external reinforcing grid such asan orthogonal array of wires, cables or similar flexible elements undertension. These reinforcing grids may also be used as bracing points forspacers which extend between adjacent wall members to keep themseparated by a predetermined distance.

A particularly advantageous system incorporating a heat exchanger ofthis description comprises an annular array of such wall membersextending substantially radially with reference to a vertical axis. Thisarrangement ensures that the liquid descending on both inner sheetsurfaces of each wall member is in contact with a thermally coactingexternal fluid such as an airstream traversing the intervening passages.The array of wall members may form part of a cooling tower including aflue centered on the axis thereof, this flue opening at its base intothe air passages between the wall members.

The gaps extending from the supply channel of each wall member towardthe inner sheet surfaces of its envelope may be constituted by series ofapertures or by throughgoing longitudinal slots. They may be at leastpartly blocked by strips or threads of capillary material, such asporous polymers or textiles fabrics, which help distribute the liquidover the upper sheet edges and thus over the entire inner sheetsurfaces.

BRIEF DESCRIPTION OF THE DRAWING The above and other features of ourinvention will now be described in detail with reference to theaccompanying drawing in which:

FIG. 1 is a transverse sectional elevation of part of a heat exchangeraccording to our invention, including a pair of spacedly juxtaposed wallmembers;

FIG. 2 is a fragmentary view of a wall member as shown in FIG. 1 butdrawn to a relatively reduced scale;

FIG. 3 is a somewhat diagrammatic top view of an annular array of wallmembers, drawn to a still smaller scale, as seen on the line III III ofFIG. 4;

FIG. 4 is a sectional elevational view of a cooling tower embodying ourinvention, taken on the line IV IV of FIG. 3; and

FIGS. 5 and 6 are detail views showing modifications of the top of thewall members illustrated in FIGS. 1 and 2.

SPECIFIC DESCRIPTION In FIGS. 1 and 2 we have shown part of a heatexchanger designed to facilitate thermal interaction be tween a flow ofhot water and a stream of atmospheric air. The heat exchanger comprisesa multiplicity of wall members 12, only two of which have been shown inFIG. 1. Each wall member 12 comprises an overhead support in the form ofa rigid tube 2 which encloses a supply channel 2b for hot waterintroduced, for example, by a pump as illustrated in FIG. 4. The tube 2is suspended from a ceiling 23, which may form part of the base of acooling tower 24 as shown in FIGS. 3 and 4, with the aid of two sets ofvertical rods or cables 4a which are anchored under tension torespective sockets 5 rising from a foundation 3 on the bottom of a basin16. Elements 4a form part of a pair of reinforcing grids each comprisinga set of cables 4b which are also held under tension with the aid ofnonillustrated springs, weights, jacks or the like. The grids 4a, 4bbear upon the outer surfaces of a pair of sheets 1 of plastic materialforming part of a flat upright envelope which in FIGS. 1 and 2 is showndraped about the tube 2 and which is also closed at its narrow sides asindicated at 1a. The open bottom of this envelope dips into a pool ofwater W in basin 16 whose level N is well above the lower edge lb of theenvelope; thus, the interior of the envelope is completely sealedagainst the atmosphere. This interior is filled by a volume of air underpressure which results in a depression of the water level N within theenvelope with reference to the outer level N.

Tube 2 is formed with two longitudinal rows of apertures 2a which openonto the inner surfaces of sheets 1 so as to distribute the incoming hotwater over the upper edge portions of these surfaces. The distributionmay be facilitated by wicks 22 extending from the apertures 2a intocontact with the sheets. Thus, the water from channel 2b descends in apair of continuous films 6 along the confronting inner sheet surfacesinto, the pool W from which it is constantly removed at the same rate,e.g., by a pump 17 as shown in FIG. 4, to maintain the level Nsubstantiallyconstant. In running down the sheets 1, the water is inintense heat-exchanging contact with an external airstream traversingthe passage 25 between adjoining wall members 12, the width of thispassage being maintained by a set of spacing bars 7 (only one shown inFIG. 1) engaging the reinforcing grids 4a, 4b on the two wall members.The spacing of the tensioned elements 4a, 4b of these grids issufficient to give the air almost complete access to the outer sheetsurfaces.

As illustrated in FIG. 5, the continuous tube 2 may be replaced by aprofile 2' of inverted-U shape separated by narrow slots 2a from a plate2'c to define a supply channel 2'b. A capillary liner 20, advantageouslyconsisting of a porous polymer such as silicone sponge, surrounds thechannel 2'b and overlies the slots 2a to help distribute the water overthe inner surfaces of a pair of plastic sheets 1 so as to form again twocontinuous liquid films 6' descending along these surfaces. In thisinstance, the sheets 1' are not integral with each other but areinterconnected in airtight fashion by the profile 2. The narrow sides ofthe envelope are formed by webs la which should be rigid enough tosupport the plates 2c.

In FIG. 6 we show another modification in which a U-shaped profile 2"constitutes a trough forming part of an upwardly open channel 2"bcommunicating via clearances 2"a with the inner surfaces of a pair ofplastic sheets 1 these clearances being occupied by capillary layers 21which may be of the same material (e.g., silicone sponge) as liner 20 inFIG. 2. The water in channel 2"b, overflowing the trough 2", penetratesthe layers 21 and descends along the inner surfaces of sheets 1 in apair of continuous films 6". The narrow sides of the envelope formed bysheets 1" are closed by,

webs 1"a.

The envelopes of FIGS. 5 and 6 are also sufficiently fluidtightto befilled with air or some neutral gas under pressure to inflate the sheets1 or 1" againstexternal reinforcing grids which have not beenillustrated in these Figures. I

FIGS. 3 and 4 show the overall construction of a heat exchangeraccording to our invention as embodied in. 1 cooling tower 24. Thistower comprises a flue 10 centered on a vertical axis 0 which is alsothe center of an annular array of wall members 12 separated by passages25 as described above. Air flows radially inwardly at the base of thetower, either exclusively on account 1 of the updraft created in theflue or with the assistance of nonillustrated blowers which could bedisposed between two annular treatment zonesZ, and Z alongside a set ofwater separators 1 1. Outer zone Z is peripher ally divided into amultiplicity of sectoral compartments, aligned with respective radialpassages 25, by piers 26 supporting the roof 23 which isintegral withthe wall of flue 10. Hot water, arriving through an inlet 14, isdelivered by pump 15 to a circular conduit or manifold 13 communicatingwith all the tubes 2 (or their equivalents as shown in FIGS. 5 and 6) ofthe multiplicity of wall members 12 which define the inner treatmentzone Z The passing airstream cools the incoming water descending,,asdescribed above, within the envelopes of wall members 12 and collectingat the bottom thereof in the annular basin 16 for extraction by the pump17. The latter feeds another annular manifold 8 serving a multiplicityof outwardly radiating nozzles 9 which, within the compartments of zone2,, disperse the water into the oncoming airstream. The water particlescollect in an annular trough 18 at the bottom of these compartments, thecooled effluent being discharged via a conduit 19. Some of theseparticles are entrained by the air flow and are removed therefrom by theseparator 1 1 which directs them into the trough 18. Thus, the airrising within flue 10 after lowering the temperature of the water has ahumidity not much different from that of the ambient atmosphere.

A variety of hydrophilic compositions are available for the plasticsheet material of the envelopes 1, 1', 1

of the wall members 12. Cellulose acetate, for example,

is suitable even though it has a rather low flow resistance. The sheetscould also consist of a relativelyhydrophobic substrate, such aspolyethyleneterephtha-= late, with a surface rendered hydrophilic byradiochemical grafting of, for example, acrylic acid as is well knownper se. It is also possible to coat the inner surfi face of ahydrophobic substrate with a layer of a relatively cheap hydrophilicmaterial such as, for example, a

cotton gauze The thickness of these sheets may be a fraction of amillimeter, e.g., approximately 0.2 mm. 7

The width of the passages 25 (as determined by the spacers 7) may be amultiple of the width of the airspace within each envelope, i.e., thedistance separating the confronting sheet, surfaces; the ratio of thewidths may be on the order of 5:1. In practice, these widths may be onthe order of centimeters, e.g., 5 cm for the passage 25 and 1 cm for thewall member 12. The height of the latter member may be on the order ofmeters, e.g., up to about 10 m, with a flue 111 ranging in heightbetween approximately and m. Such I a heat exchanger may have a coolingcapacity of about Q(Kcallhr) rate of heat dissipation ll ll Re(m=/hr)volumetric output (i.e. size of sheet in in times kinematic viscosity inm/hr) AT(C) temperature difference between fluid inlet and outletAt,,.(C) mean logarithmic temperature difference between the two fluidsK(watts/m C) overall heat-transfer coefficient for airstream.

These numerical values, taken with the air passing in counterflow to thewater and with the two flows shielded from the ambient atmosphere, arelisted in the table below:

channel on said support, the latter being provided with gaps leadingfrom said channel to said airspace and terminating adjacent the innersurfaces of said sheets for distributing said first fluid substantiallyuniformly along upper edge portions of said sheets and letting saidfirst fluid descend in a continuous film along said inner surfaces;

collector means for said first fluid at the bottom of said envelope; and

circulation means for driving said second fluid through passages betweensaid wall members in contact with the outer surfaces of their sheets forthermal interaction with the films on the inner surfaces thereof.

2. A heat exchanger as defined in claim 1 wherein said envelope issealed against the atmosphere at its top and sides, said collector meanscomprising a basin filled with said first fluid to a level above thebottom of said envelope, said wall members being filled with a gas TestQ re AT AT K a (Kcal/hr) m hr (C) ("c") (watts/m (watts/m water airwater air water air C) C)air It will be noted that the measured valuesfor the overall heat-transfer coefficient K are very close to thetheoretical values a for the transfer on the air side. This indicatesthat the heat exchange between the fluids is essentially determined bythe heat transfer between the plastic sheet and the air. The value of Kin the neighborhood of 18 watts/m C corresponds to the requirements ofthe contemplated use in a cooling tower.

Aside from the possible effect of the irrigation of the airstream inzone Z the heat exchanger according to our invention operates withoutany increase in the humidity of the ambient air. The wall members 12 canbe manufactured at a cost substantially lower-than that of finned tubesof like capacity as conventionally used in dry cooling towers. Thepresence of the tensioned grids 4a, 4b allows the spacing of the sheetsto be made substantially uniform throughout each wall member,

with only a slight undulation of the sheet profiles,

owing to the gas pressure maintained in these envelopes. This gaspressure is constant for all points of the envelope and independent ofthe air flow through passages 25.

We claim:

1. A heat exchanger for establishing thermal contact between a first anda second fluid of different temperatures, at least said first fluidbeing a liquid, comprising:

a plurality of spacedly juxtaposed hollow wall members each including agenerally horizontal elevated support and an envelope of plastic sheetmaterial suspended from said support, said envelope comprising a pair ofconfronting sheets separated by an airspace;

conduit means for said first fluid terminating in a under pressuremaintaining their envelopes in an inflated state.

3. A heat exchanger as defined in claim 2 wherein said wall members arefurther provided with a reinforcing grid along the outer surfaces ofsaid sheets.

4. A heat exchanger as defined in claim 3 wherein said grid consists ofintersecting flexible elements under tension.

5. A heat exchanger as defined in claim 3, further comprising spacingmeans between adjacent wall members bearing upon the grids thereof.

6. A heat exchanger as defined in claim 1 wherein said wall members forman annular array and extend substantially radially with reference to avertical axis.

7. A heatexchanger as defined in claim 6 wherein said second fluid isair, further comprising a flue centered on said vertical axis, said flueopening at its base onto the passages between said wall members.

8. A heat exchanger as defined in claim 1 wherein said first fluid iswater, said channel being provided with capillary means at least partlyblocking said gaps for helping distribute the water over said upper edgeportions.

9. A heat exchanger as defined in claim 1 wherein said sheets have athickness of a fraction of a millimeter, said wall members and saidpassages have widths on the order of centimeters, and said wall membershave a height on the order of meters.

10. A heat exchanger as defined in claim 9 wherein the width of saidpassages is several times that of said wall members.

1. A heat exchanger for establishing thermal contact between a first anda second fluid of different temperatures, at least said first fluidbeing a liquid, comprising: a plurality of spacedly juxtaposed hollowwall members each including a generally horizontal elevated support andan envelope of plastic sheet material suspended from said support, saidenvelope comprising a pair of confronting sheets separated by anairspace; conduit means for said first fluid terminating in a channel onsaid support, the latter being provided with gaps leading from saidchannel to said airspace and terminating adjacent the inner surfaces ofsaid sheets for distributing said first fluid substantially uniformlyalong upper edge portions of said sheets and letting said first fluiddescend in a continuous film along said inner surfaces; collector meansfor said first fluid at the bottom of said envelope; and circulationmeans for driving said second fluid through passages between said wallmembers in contact with the outer surfaces of their sheets for thermalinteraction with the films on the inner surfaces thereof.
 2. A heatexchanger as defined in claim 1 wherein said envelope is sealed againstthe atmosphere at its top and sides, said collector means comprising abasin filled with said first fluid to a level above the bottom of saidenvelope, said wall members being filled with a gas under pressuremaintaining their envelopes in an inflated state.
 3. A heat exchanger asdefined in claim 2 wherein said wall members are further provided with areinforcing grid along the outer surfaces of said sheets.
 4. A heatexchanger as defined in claim 3 wherein said grid consists ofintersecting flexible elements under tension.
 5. A heat exchanger asdefined in claim 3, further comprising spacing means between adjacentwall members bearing upon the grids thereof.
 6. A heat exchanger asdefined in claim 1 wherein said wall members form an annular array andextend substantially radially with reference to a vertical axis.
 7. Aheat exchanger as defined in claim 6 wherein said second fluid is air,further comprising a flue centered on said vertical axis, said flueopening at its base onto the passages between said wall members.
 8. Aheat exchanger as defined in claim 1 wherein said first fluid is water,said channel being provided with capillary means at least partlyblocking said gaps for helping distribute the water over said upper edgeportions.
 9. A heat exchanger as defined in claim 1 wherein said sheetshave a thickness of a fraction of a millimeter, said wall members andsaid passages have widths on the order of centimeters, and said wallmembers have a height on the order of meters.
 10. A heat exchanger asdefined in claim 9 wherein the width of said passages is several timesthat of said wall members.